1 The survey of the literature pertaining to this review was concluded in December, 1951.
2 Lalor Foundation Fellow.
3 The author is greatly indebted to Dr. James Bonner, Dr. A. W. Galston, Dr. K. C. Hamner, and Dr. J. L. Liverman for numerous valuable suggestions in preparing this review, and to these and many other colleagues for permission to use unpublished data.

Flowering can be separated into the following major stages: (a) floral initiation (the differentiation of floral primordia); (b) floral organization (the differentiation of the individual flower parts); (c) floral maturation, consisting of several processes, some of them concurrent or overlapping (growth of the flower parts, differentiation of the sporogeneous tissues, meiosis, pollen and embryo sac development); and (d) anthesis.

In this review the physiology of all four stages has been considered. The treatment of the three later stages, however, has been combined and is limited to those changes which involve sporophytic tissues only and which are integrating elements of the flowering process. For this reason, the physiology of meiosis, the entire pollen physiology, and the development of flower pigmentation and of any accessory flower parts are excluded. Through-out the treatment, stress has been laid upon the mechanism of the developmental changes, that is, on those physiological and biochemical processes which actually initiate or control development. Work of a purely descriptive nature has been referred to only when it is apt to shed some light on mechanisms as well. The review is therefore by no means a complete survey of the literature, even of the most recent vintage.

FLORAL INITIATION

Floral initiation and the later stages of flowering.—Of the various stages of flowering, floral initiation is by far the most fundamental one, for it marks the actual switch from vegetative to reproductive development. Most of the work on the physiology of flowering has been concerned with this stage. This is so even though in many cases some later stage (anthesis or the appearance of visible buds) has been observed, for the variations in these stages have been but projections of variations in the onset of floral initiation. In this connection, however, an important methodological question must be raised. Variations in the onset of floral initiation frequently persist unchanged throughout the following stages of flower development; but the initiation of floral primordia and their subsequent fate are still separate phenomena and may show a different dependence on one and the same condition. [266] Furthermore, the time of floral initiation depends on the rate of the preceding vegetative growth, and conditions which influence this rate may cause differences in the time of flower formation without having affected initiation in a specific manner. It therefore becomes imperative to use a criterion which permits clean separation between specific and nonspecific effects in floral initiation. Such a criterion is the relative amount of the preceding vegetative growth, as expressed by the number of leaves (or leaf pairs or whorls) preceding the first flower. This approach was first used in a consistent manner by Purvis (185, 190) in Secale; it is readily applicable to any plant with terminal flowers or with a clearly demarcated terminal inflorescence and also to many plants with a systemic habit of flowering, for example, Gossypium (121). Presence or absence of differences in the leaf numbers tells us whether initiation has been affected specifically or not, regardless of whether or not there are differences in the time of appearance of floral primordia, buds, or open flowers.

Photoperiodism and vernalization.—In numerous plants floral initiation shows a highly specific dependence on certain environmental conditions. These conditions may be quite different from those which are favorable for growth and other vegetative functions of the plant. The bulk of our information on the physiology of floral initiation has been derived from the study of such cases, for in plants lacking a specific environmental control of flowering it is extremely difficult to modify this process experimentally and thus make it accessible to analysis. The two conditions which most frequently control floral initiation in a specific manner are daylength and low temperature. The control of physiological processes by daylength or photoperiod—of which flower formation is but one case, although the most common and the most spectacular one—is called photoperiodism. The low-temperature requirement in annual herbaceous plants, in which it has been studied most thoroughly, has been termed vernalization.

The basic facts of photoperiodism and vernalization are well known. The terms and abbreviations which follow will be used in this review.

Long-day plants (LDP): plants in which flowers are formed only in daylengths exceeding a definite value or in which flower formation is promoted in such daylengths.

Short-day plants (SDP): plants in which flowers are formed only in daylengths below a definite value, or in which flower formation is promoted in such daylengths.

Recently, a reclassification of plants with respect to their daylength dependence has been suggested, involving new names for all of the photoperiodic types (44). It is difficult, however, to see the point of such a proposal. The accepted terms are not only highly suggestive of what they describe, but also convey all that can be conveyed in the present state of our knowledge. It is essential, however, to use the recognized terms with greater accuracy. Thus, definitions of LDP and SDP, which are based on absolute or on relative specifications of daylength, are bound to in imperfect, since the inductive ranges of the two types overlap and critical daylength is a character [268] which is specific for every single species or variety. Even the definitions given above, which are unequivocal where they are applicable, are not quite all-inclusive. In some plants, any change of daylength throughout the entire range will promote or retard flower formation. These plants, thus, do not possess a pronounced critical daylength (see Fig. 1), and are not adequately covered in a classification based on this concept. This question will be returned to later (see p. 284).

PHOTOPERIODISM

The floral stimulus arising in photoinduction.—Both daylength and low temperature have a pronouncedly inductive mode of action. The effect appears with some delay and, in fact, frequently after the causative condition is no longer operative. In vernalization this is frequently the rule, for during the actual low-temperature treatment (the thermal induction) the growth and development of the plant may be completely suspended. In Photoperiodism, flowering usually occurs most rapidly with continuous inductive treatment, but some flowering response can be obtained with inductive periods far too short to permit any flower formation in the course of the actual treatment. This aftereffect of limited periods of inductive treatment is called photoperiodic induction. In addition, the effect of photoinduction in LDP is not reduced by interruptions with periods of short-day conditions (79, 123, 132, 169, 208), and it is possible to obtain flower formation in an LDP by "fractional induction," that is, by exposing the plant to an induction period insufficient to cause a flowering response and repeating this subinductive treatment alter a period of short-day conditions.

Thus, both in low-temperature and in daylength action we have to distinguish between the immediate action of the inductive conditions and the subsequent changes which result in actual floral initiation, and we have to try to separate these different changes. This is particularly urgent in photoperiodism for it seems that the opposite conditions of long and short days result, in different plants, in one and the same response, i.e. flower formation. To understand the daylength control of floral initiation, we must start by establishing whether or not the final changes which are responsible for this process are identical in LDP and SDP.

It has been known for some time that floral initiation in LDP and in SDP depends on the photoperiodic treatment of the leaves and that it is immaterial whether or not the sites of the actual response, the growing points, are subjected to the inductive conditions (17, 28, 31, 78, 81, 82, 94, 96, 108, 130, 133, 159, 160, 229). Also, the effect of temperature on the flowering response is the same if either entire plants or the leaf blades alone are kept at different temperatures during photoinduction (176, 178), whereas differential temperature treatment of the petioles or the stem tips becomes effective only at much lower temperatures (19). The immediate action of daylength takes place in the leaves and the effect is transmitted to the growing points. [269]

Two basically different possibilities can be visualized: (a) the inductive daylength promotes floral initiation; and (b) the noninductive daylength inhibits it. The former alternative implies that an LDP or an SDP is incapable of floral initiation unless it is photoinduced. The latter alternative implies that the plant is potentially capable of floral initiation under any daylength, but that initiation is secondarily suppressed by noninductive daylength conditions. The evidence summarized below shows clearly that a flower-promoting effect of the inductive daylength is an essential factor in photoinduction. First, favorable temperature conditions during induction promote floral initiation, unfavorable temperatures delay or suppress it (see p. 275). Second, if photoinduction is limited to part of a plant, flowers can also be formed (under certain conditions) in the noninduced parts. In extreme cases, photoinduction of a single leaf is sufficient, even if all other leaves of the plant are maintained on the noninductive conditions (81, 82, 94, 229). Finally, if a plant is photoinduced and then is grafted to an individual kept on the noninductive conditions, the latter plant will also initiate flowers (Table I. The graft partner capable of flower formation is called donor, the partner not capable of flower formation by itself is called receptor).

All these findings show that under the influence of the inductive daylength conditions, changes take place in the plant which actively promote floral initiation. In many cases, a single leaf is sufficient as the donor in a graft. The leaf can be detached from a plant and can be photoinduced while growing as a cutting [Lona (140)]. This fact shows that all the processes of floral initiation which are under immediate daylength control can be completed within the leaves—and the leaves alone. We can conclude that floral initiation in LDP and in SDP is determined by a floral stimulus which is generated in the leaves under the influence of photoinduction and is then translocated to the growing points.

The question whether there exists a comparable, that is a transmissible, flower-inhibiting effect of the noninductive daylength conditions, will be considered later. At present we want to ask ourselves what the relationship is between the floral stimuli of LDP and SDP. Attempts to identify the stimuli chemically have met with failure (see p. 287); therefore, only an indirect approach is possible, again by way of grafting. Grafts can be made not only between induced and noninduced plants of the same response type, but with equal ease between plants belonging to the opposite response types. In such experiments (see Table 1) it becomes evident that the stimulus from an LDP can also cause flower formation in an SDP and vice versa. The stimuli of the two photoperiodic types are interchangeable.

This interchangeability, however, is not yet a conclusive proof of identity. It is unlikely that we are dealing with two entirely unrelated and independent stimuli which have the same physiological activity; but it is conceivable that there are two independent complementary stimuli or that one stimulus is formed in two stages. In that case we can visualize that in LDP [270] the first, and in SDP the second, of these stimuli or stages is produced only under the inductive conditions, whereas in either case the other stimulus or stage is independent of daylength. To check these possibilities one must establish whether donors of the one response type cause floral initiation in receptors of the other type only if they are induced themselves. Such experiments have so far been made only in one case, using Nicotiana tabacum var. Maryland Mammoth as SDP and N. silvestris and Hyoscyamus niger as LDP (105, 121, 125). With Hyoscyamus as receptor, flower formation occurred only with short-day treated donors. In N. silvestris, some response was observed both with induced and with noninduced donors, but the entire effect may be nonspecific (see 125). Maryland Mammoth receptors flowered only when the Hyoscyamus and N. silvestris donors were maintained on long days (see Table II). While an extension of the experiments is desirable, the results indicate full identity of the floral stimuli of LDP and SDP.

+The table lists the grafts in which daylength-dependent plants kept on noninductive conditions (the receptors) were brought to flower formatin by graft union to plants capable of flower formation (the donors). As donor, either a day-neutral plant, or a daylength-dependent plant photoinduced either before or after the grafting can be used.
*Little daylength effect in floral initiation.
**Short-day strains.
#Day-neutral strains.
%Classification doubtful.

[271]

TABLE II
EFFECT OF INDUCED AND NONINDUCED DONORS UPON RECEPTORS OF THE OPPOSITEPHOTOPERIODIC RESPONSE TYPE
(After 105, 121, and 125)

Light and darkness in the photo periodic response of long-day plants.—The foregoing discussion shows that the immediate flower-controlling effect of daylength is localized in the leaves and that it results in the formation of a floral stimulus which is alike in LDP and SDP. The difference between the two response types must be sought in those changes which photoinduction causes to take place in the leaves. Daylength or, as it was described by Garner [272] and Allard, the daily light-dark ratio, is not an elementary environmental condition, like light alone, temperature, humidity, etc., but is a composite of light and darkness. The first task in the elucidation of photoperiodic responses will be the disentangling of the respective roles of the two conditions. The principal material available for this task stems from two types of approaches: the use of light-dark cycles of different total length and the use of differential experimental conditions in the light and in the dark periods. To establish the individual significance of the two periods they must be varied separately, but as long as one works with the natural 24-hr. cycles the length of the two periods can be varied only simultaneously. Therefore, one must change the total length of the cycle or subject the plants to one set of experimental conditions in the light periods and to another set in the dark periods. Another approach which has proven to be very valuable is the application of small amounts of light in the dark periods, given either as extended periods of illumination with very low light intensities, or as short-time interruptions with light.

In the case of LDP, both indirect and direct evidence indicates that an essential factor in their photoperiodic response is an inhibitory effect of long dark periods, an effect which tinder short-day conditions prevents the formation of the floral stimulus.

4) Under "short" and "long" light and dark periods, any periods are understood which are shorter or longer, respectively, than the light or dark period at the critical daylength of the plant in question.

The indirect evidence follows. (a) LDP flower in continuous light and no kind of light-dark alternation has been found to be superior to the continuous light regime (43, 79, 114, 119, 123, 132, 164). Thus, dark periods do not have any indispensable positive part in the formation of the floral stimulus in LDP. (b) LDP stay vegetative if short light periods are combined with long dark periods. If, however, short light periods are given together with short dark periods, flower formation takes place (3,121, 208).4 The critical factor in short-day action is not the shortness of the light periods but the length of the dark periods. (c) In most LDP flowering response and temperature seem to be negatively related. Floral initiation is promoted and the critical daylength is lowered as the temperature is decreased (109, 123, 197), the decisive role in this effect being played by the temperature of the dark periods (114, 123, 174). The inhibitory effect of dark periods seems to increase with increasing temperature and floral initiation is suppressed in shorter periods of time. The seemingly reverse temperature relationship which has been observed in a few LDP [Rudbeckia bicolor, Bouvardia humboldtii (167, 197, 210)] need not be a true exception to the rule, for the inverse relationship between temperature and floral initiation in LDP may hold only for a certain range of temperature and may break down if these limits are transgressed (132).

The direct evidence is based on work done with the LDP, Hyoscyamus niger. Flowers in this species are promptly initiated under short-day [273] conditions if the plants are kept continuously defoliated; the rate of floral initiation in defoliated plants is the same in long and short days (117, 122, 123). This demonstrates not only that the formation of the floral stimulus is actively suppressed in short days, but also that the inhibitory action is seated in the leaves, while the axis tissues are free from it. It is also clear that under short-day conditions the leaves are capable of preventing the formation of stimulus in the axis as well. In other words, the inhibitory action extends over some distance.

Now arises the question of what the exact function of light is in the formation of the floral stimulus in LDP. Does it only remove the adverse effects of dark periods or does it, in addition, have some positive, promotive function of its own? At first glance, the latter alternative seems to be the right one. Floral initiation can be induced in LDP by supplementing the light periods of short days with light of very low intensities (48, 64, 230, 231) or by interrupting the dark periods of short days with brief periods of light (15, 23, 49, 169, 180, 214, 215). The energies necessary for complete annihilation of the effect of long dark periods lie, although apparently varying within rather wide limits in different plants, between 100 and 1000 f.c. min. (foot candles X minutes). A significant effect may be obtained with 10 f.c. min. On the other hand, the light energies required for the effectiveness of the light periods fall in the range of photosynthetically active light. In sugar beets (Beta vulgaris), for example, the minimum is about 700 f.c. of continuous light (169) or approximately 1 X 106 f.c. min. per 24 hr. It appears that we are dealing with two distinct light actions, one which promotes the formation of the floral stimulus directly and requires high amounts of light energy and another which removes the inhibitory effects of darkness and requires small amounts of light energy.

However, the relations of light to the formation of the floral stimulus in LDP are apparently more complex. Hyoscyamus forms flowers upon defoliation not only under short-day conditions, but also in continuous darkness (117, 122, 123). Thus, if no inhibition is present, the formation of the stimulus is not dependent on light at all; light periods are necessary only because this process is normally suppressed in dark periods. It is true that to date a comparable effect of defoliation has not been observed in other LDP (42, 123, 125, 132). But in these cases the defoliated plants failed to produce flowers not only in short days or in continuous darkness, but also under long-day conditions. Therefore, these negative results do not invalidate the positive ones which have been secured with Hyoscyamus. It seems that either the generation of the floral stimulus itself in most LDP takes place only in the leaves or that to carry out this function (or perhaps, in some cases, to carry on a sufficient amount of growth, which is of course the prerequisite for the production of any new structures) these plants need a continuous supply of some material which is formed only in light and in the leaves. Hyoscyamus is unique only in that it is capable of carrying out these functions at the expense of stored material alone. It is important to note that [274] this plant also forms
flowers upon defoliation only after having attained a certain size and having
produced a well developed storage root. Intact plants are sensitive to
photoinduction at a much earlier age.

The specific action of
light in the photoperiodic control of floral initiation in LDP is, then, the
counteraction of the inhibitory effect of dark periods in the formation of the
floral stimulus. This effect is accomplished by small amounts of light energy.
High-intensity light has only some preparatory role and does not enter directly
into the formation of the floral stimulus.

This latter concept is
supported by several additional facts. If Hyoscyamus is grown on 48-hr. cycles, floral initiation
occurs with shorter periods of light per cycle than it does in 24-hr. cycles (9
versus 11 hr.), although the dark periods are naturally much longer (50). In
the LDP Spinacia (spinach),
flower formation can he obtained in individuals raised in total darkness (on
sugar-containing media) (70). It is possible that the inhibitory effect which
appears in the dark periods is ultimately dependent on the normal development
and functioning of the leaves, that is, on the presence of high-intensity light
periods.

The question which
remains now is how the counteracting effect of the low-intensity light is
achieved. In considering this question, two most essential facts must he taken
into account: (a) while LDP remain vegetative on short-days, they form flowers
with long light periods even if the dark periods are simultaneously extended by
a much greater factor (3, 50, 132, 208); and (b) as noted earlier (p. 268), an
interruption of the photoinduction of an LDP does not reduce its effectiveness.
It thus seems that the inhibitory action of dark periods is limited to the immediately
adjacent light periods. This may be caused partly by the fact, discussed in the
preceding paragraph, that the
effectiveness of the dark periods is not unlimited, but declines if the periods
become too long. However, fractional induction could be explained on this basis
only if the maximum of the dark-period effect were already reached at the
critical daylength. Since the dark-period effect is increased by higher
temperature, this does not seem to he the case. Two assumptions can he made:
(a) a dark period tends to destroy the changes which have taken place in the
preceding light period, but if the light period is extended beyond a certain
limit, further changes take place which are insensitive to the dark-period
effect; (b) some adverse effect of the preceding dark period must be eliminated
in the course of a light period before the formation of the floral stimulus can
set in. The first alternative implies that the formation of the floral stimulus
proceeds in two stages and that the second cannot he entered until the first
has attained a certain level (208). The second alternative implies that the
floral stimulus itself is insensitive to the dark-period effect from the
beginning and is being produced to the extent that the darkperiod effect has
been sufficiently reduced. This alternative seems more plausible. It is also
supported by the observation that supplementary low-intensity light (see above)
is more effective in promoting floral initiation if given prior to the
high-intensity light period than if given afterwards (64). This may mean that
the initial low-intensity light removes the dark-period effect [275] and thus enables the
plant to utilize fully the high-intensity light. The evidence, however, is not
yet conclusive. If the light action follows the dark action, it must be
assumed, because of the defoliation experiments in Hyoscyamus,that the dark action involves the production of a transmissible,
inhibitory material. This possibility has not been studied so far. As long as
it has not been proven one may consider that the inhibiting action of darkness
is localized entirely within the leaves and that the distance action which is
evident in Hyoscyamus iscaused by the diversion of
some material necessary for the production of the floral stimulus from the axis
tissues to the leaves (cf. 123). An inhibitory effect of short-day leaves has
also been noted in other LDP in experiments with localized photoinduction (38,
39, 229); but this may be interpreted either way and, in addition, may be
caused not by an inhibition of the formation of the stimulus, but of its
translocation from long-day leaves to growing points (see p. 282). As long as
these questions are not settled, the exact mode of interaction of light and
darkness in the photoperiodic response of LDP remains an open question.

5It seems that in some
SDP the effect of the dark periods does not decline as the periods are
extended but continues to increase, although apparently at a slow rate. Perilla plants, which will not respond to a single
optimal 24-hr. induction cycle, form flowers if exposed to a single long dark
period (130 hr. or more). If, however, the plants are given several 24-hr.
cycles, the total number of dark hours required for photoinduction may be as
low as 36 (139).

Light and darkness in short-day plants.—The significance of light and darkness
in SDP becomes evident from the following results. (a) SDP initiate flowers
only if they receive light-dark cycles with sufficiently long dark periods. If
the dark periods are reduced below a certain limit the flowering response will
decline and will ultimately fail; however, the response is also reduced if the
light periods are reduced, even if the dark periods are maintained at an
optimum level (see data in 3, 79, 81, 164, 198). Furthermore, if differential
treatment is applied in the light and the dark periods of photoinduction (see
p. 272), the response is affected in a similar manner: it is promoted by
favorable conditions (favorable temperatures) and is reduced or suppressed by
unfavorable ones (low or excessively high temperature, application of
narcotics) (81, 87, 143, 145, 176). Both light and dark periods are evidently
needed for the formation of the floral stimulus in SDP.(b)In some SDP [Xanthium (79); Chenopodium amaranticolor (139)] a single photoinductive cycle may be
sufficient to cause floral initiation. In this case, the light period has to
precede the dark period (79). Thus, light periods of sufficient length are
needed to render inductive dark periods effective. (c) If the light periods are
kept at a constant, optimal level and the dark periods are extended, the
flowering response is reduced (3, 198). The beneficial effect of a light period
seems to be limited; when it is used up, the dark period effect, which has
already built up, apparently begins to deteriorate. This deterioration must be
rather slow, for the decline of the response with extension of the dark periods
is gradual (3, 198), and complete failure occurs, if at all, only with very
long dark periods [in Kalanchoë blossfeldiana, for example, 88-112 hr (98)].5[276](d) If an SDP can be
photoinduced with a single light-dark cycle, there is no upper limit to the
light period; but in SDP that require more than one induction cycle, extension
of the light periods leads to a rapid decrease of the flowering response (3,
79, 198). Also, fractional induction seems to be not possible in SDP, at least
not without loss in effectiveness (14, 98, 121, 143). Thus, while light is
required to make the following dark period effective, the effect of a dark
period is reduced and can be nullified in the following light period. In other
words, we are dealing in SDP with two antagonistic light effects. The formation
of the floral stimulus depends on two sets of changes, one requiring light
energy, the other inhibited by light. The light-inhibited changes cannot set in
before the light-requiring changes have become effective; but they must be, in
their turn, protected from too long light periods. The floral stimulus is
produced or persists in SDP only if the plants are subjected to regular
alternations of appropriate light and dark periods.

The inhibitory action
of light on SDP is accomplished by the same minute quantities of light as is
the low-intensity light action on LDP. These quantities may be given as
low-intensity supplementary light (64, 230, 231) or as brief light
interruptions in the dark periods (79, 81, 84, 181, 192, 206, 214, 215). This
light action is very similar to the low-intensity light action on LDP, but its
effect with regard to floral initiation is just the opposite.

The minimum duration
of the light periods and the minimum amounts of light which are required to
make them effective, show an amazing variation between species. Some species
need light periods comparable to the minimum light periods of some LDP. Such
plants have sometimes been considered as a separate photoperiodic group
[intermediate or middle-day plants (2, 164)]; but the demarcation between these
and the usual SDP is a gradual one. Several typical SDP have minimum light
periods between two and five hours (164), and the light intensities required in
the light periods of SDP are comparable to those needed in the light periods of
LDP (cf. 17, 79, 145). In Kalanchoë blossfeldiana, however, as little as one second of daylight
proves sufficient to secure a flowering response which is not greatly inferior
to the optimal response obtainable [Harder & Gümmer (88)]. In Xanthium and in Perilla, flower formation can be induced by transferring
the plants to continuous darkness (79, 139). In Perilla this treatment is effective even if it is
preceded by an extended light period of very low light intensity [Lona (139)].
Extension of the light periods in Kalanchoë results in an increase of the response only
after periods of approximately 30 min. have been reached. These remarkable
facts suggest that the promotive light action in floral initiation of SDP may
be composed of two different effects: one which requires only small amounts of
light energy and another which becomes effective only with rather high amounts
of light energy. Time high-intensity light effect can apparently be dissociated
from the actual processes of floral initiation, at least in some short-day
species, and can be replaced by storage material. Similar to the high-intensity
light action of LDP, this effect is of a preparatory nature. This idea is again
supported by the observations that Perilla[277] plants respond to the
dark treatment only after they have reached a certain age and that the
effectiveness of the extremely short light periods in Kalanchoë is consistent only in plants which are in good
vegetative condition. The promotive low-energy light effect, in turn, seems to
have some immediate function in the formation of the floral stimulus, for if
the one-second light periods in Kalanchoë are omitted and the plants are kept in continuous
darkness, they stay strictly vegetative (88).

Chemical changes
involved in the formation of the floral stimulus—The analysis of the photoperiodic responses of
LDP and SDP enables us-to distinguish several well defined partial processes in
the formation of the floral stimulus. We can distinguish four such processes in
LDP: a preparatory high-intensity light process (L-I); a light-independent
process which results in the actual appearance of the stimulus (L-II); a dark
process antagonistic to L-II (L-III); and a low-intensity light process
antagonistic, in its turn, to L-III (L-IV). The following partial processes are
well established in SDP: a light process (S-I) and a dark process (S-II), both
of which are necessary for the stimulus to be produced; and a low-intensity
light process antagonistic to S-II (S-III). If antagonistic relationships are
expressed by arrows, the sequence of the processes can be written as follows:

As discussed above,
S-I consists quite likely of a preparatory, high-intensity light process
comparable to L-I and of a low-energy light process which is directly involved
in the production of the floral stimulus. L-II may have to be subdivided into
two separate processes (see p. 274), only the first of which would be subject
to the action of L-III.

The separation of the
whole process of the formation of the floral stimulus in LDP and SDP into a
series of partial processes provides us with a basis for biochemical approaches
to the problem. Before this separation had been achieved, there was practically
no way to establish if any chemical changes following photoinduction were
related to floral initiation in a causal or in an incidental manner. The
extensive studies of such changes which were carried out in the earlier period
of research in photoperiodism, or were carried out later without taking into
account the recognition of the partial processes, have yielded next to nothing
towards a biochemical understanding of the photoperiodic responses.

The possibility and
necessity of separating photoperiodic responses into partial processes have
been recognized only in the course of the last 10 or 12 years, beginning with
the work of Hamner & Bonner (81) and Hamner (79) with SDP and of Lang &
Melchers (123) with LDP. In view of this short period of time it is
understandable that the amount of conclusive biochemical information on the
individual partial processes is still very limited. What we [278] do know concerns the
high-intensity light process of LDP (L-I), the promotive light process of SDP
(S-I) and the inhibitory dark process of the former (L-III). In addition, we
have some evidence that the auxin level in the leaves plays some important part
in the photoperiodic induction, at least in SDP. This question will be
considered in a separate section.

The high-intensity
light processes of both photoperiodic types seem to be identical with
photosynthesis or to be closely tied in with it. This fact is suggested in the
first place by the order of magnitude of the light intensities which are
required for these processes, and is supported by the following evidence: (a) Hyoscyamus plants initiate
flowers under noninductive conditions if supplied with sugar (123); and (b) in Xanthium, floral initiation can be
induced by a single dark period if the plants, instead of being subjected to a
high-intensity light period, are fed with sugar (12). The experiments with Spinacia (p. 274) and with defoliated Chenopodium (p. 285)may also be cited in this connection.

If the high-intensity
light processes of LDP and SDP are identical with photosynthesis, their
function is evidently restricted to securing the substrates which furnish the
energy required in the formation of the floral stimulus and their relationship
to this latter process would he indirect-in fact, rather remote. This is in
agreement with the findings, discussed earlier, that in some plants the
high-intensity light effect can he separated from the actual formation of the
stimulus and can be replaced by storage material. In LDP this nonspecificity is
particularly evident, for the effectiveness of the inhibitory dark process (L-III) seems also to depend ultimately on the presence of adequate light periods
(see p. 274). L-I would seem to supply the substrates for any
reactions involved in flower formation, whether promotive or inhibitive.

As to what part of the
energy metabolism is involved in the generation of the floral stimulus,
we have only one experimental indication. In Xanthium, the light periods may
be replaced not only by feeding sugar, but also by organic acids of the Krebs
cycle (131). An additional supply of these acids during the light periods of
photoinduction increases the flowering response (106). Thus, the essential factor may be the
transformation of pyruvic acid rather than the preceding glycolytic sequence.

As to the light which
promotes floral initiation in SDP (low-energy effect), we have to date again
only one indication of the direction in which its effect might be sought. It
has been found that photoinduction is effective in SDP only if the atmosphere
in which the plants are maintained during the light periods contains CO2 (177).
This seems to be a further proof that the high-intensity light process is
identical with photosynthesis. But Since in Kalanchoë only one second of light is required, this is
probably not the sole explanation. In Kalanchoë, too, CO2 is necessary for photoinduction. Moreover,
in localized inductive treatment the short-day parts of the plant must be
supplied with CO2 directly; the presence of CO2 over large long-day areas is of no avail (86). This necessity suggests that
some CO2 fixation not identical [279] with photosynthesis
may be involved in the formation of the floral stimulus and that to carry out
this fixation the plants must receive at least a small amount of light. If
would be most interesting to know whether CO, must be present during the
one-second light periods themselves or whether it might be given shortly
afterwards. Whether the effectiveness of organic acids has a special meaning in
this connection also remains to be seen.

Process L-III is tied
in with oxidative reactions, for floral initiation in LDP can he caused under
short-day conditions by keeping the plants during the dark periods (or part
thereof) in an atmosphere of nitrogen (48, 152, 238).

Auxin and
photoperiodic induction.—The
roleof auxin in the
photoperiodic induction of SDP is basically clear. First, flower formation by
SDP can be suppressed under short-day conditions by treating the plants with
auxin or synthetic growth-regulating substances (13, 90, 131, 141, 201, 218)
and can be induced under noninductive conditions by treating with auxin antagonists
(8, 131) or with ethylene chlorhydrin (105), an agent which also seems to lower
the auxin content of plants (cf. 158, 213). Thus, floral initiation depends on
a reduction of the physiological auxin level in the plant. Second, leaves of a
SDP which were treated during photoinduction with an auxin are not effective if
used as donors to receptor plants maintained on long days (13). Thus, the
crucial factor seems to be the auxin level in the leaves during photoinduction,
and the effect of high auxin levels seems to consist in an inhibition of the
formation of the floral stimulus. Third, the effect of auxin on photoinduction
is most pronounced if the application is made at the beginning of the dark
period (9). If the application is made following the dark period (9, 131) or
the end of the photoinductive treatment (131, 201), the effect is much smaller
or nonexistent. Thus, the auxin level seems to be specifically associated with
the functioning of the dark-period process of SDP (S-lI) or with its immediate
outcome, and the effect of inductive dark periods seems to involve a lowering
of the auxin level in the leaves. The later stages of flower development seem
to require an increase in the auxin supply, for in Xanthium, antiauxin-induced inflorescence primordia may
fail to develop unless the plants are treated later with auxin (131).

In what manner
inductive dark periods affect the auxin level in the leaves, and in what exact
manner this level is involved in the formation of the floral stimulus, is
entirely a matter for speculation. An essential step in the elucidation of this
problem would be a direct determination of the changes in the auxin level of a
leaf in the course of an inductive dark period and of photoinduction in
general. To date, only some comparisons of the auxin content in short-day and
in long-day treated plants are available. The do indicate that the amount of
extractable auxin may be less under short-day conditions (9, 12, 233, 237);
however, wherever in these determinations the relationship between the activity
and the dilution of the extracts was checked, it was not found to be linear.
These results, therefore, are not reliable. The apparent reason for the lack of
linearity lies in the fact that the leaves of numerous plants contain some auxin
inhibitor. Its presence not only prevents auxin [280] determinations
by the classical methods, but may also complicate the entire issue, for the
physiological auxin level in a tissue may depend, not only on the amount of
auxin itself, but also on the amount of auxin antagonist present. It is to be
hoped that the use of modern separation techniques in auxin analysis (103, 129)
will help us to resolve this difficulty.

6A general
discussion of the effects of auxin in flower formation is given by Bonner &
Bandurski in another article in this volume (pp. 59-86) and also in Bonner
& Liverman (11).

The question as to
what role, if any, auxin has in the photoperiodic control of floral initiation
in LDP is an entirely open one. Most LDP grow in the vegetative state as
rosettes. Stem elongation coincides with floral initiation and may, in fact, be
slightly ahead of it, even in LDP in which the further development of flowers
may proceed without stem elongation [certain Rudbeckia species (77, 91, 167)]. A close
interrelationship between the two processes and thus between floral initiation
and auxin is indicated. Occasionally, however, stem elongation has been
observed in rosette-type, long-day plants without any flower formation,
although the conditions causing this occurrence are not clear (123), and in
some LDP [species of Urtica, Anagallis, Circaea, etc. (cf. 44, 137)] stem elongation is quite
independent of floral initiation and takes place both in long- and in short-day
conditions. The results of auxin applications and of experimental treatments
which decrease the auxin content of plants (X-ray and ultraviolet irradiation)
are not less controversial. In Silene armeria, auxin treatment seems to be capable of inducing
floral initiation under noninductive conditions (131). In Hyoscyamus the results have so far been negative, but the
periods of treatment may have been too short (49). Under long-day conditions,
flower formation was delayed by auxin application in one LDP (Calendula
officinalis) and was speeded
by ultraviolet irradiation in two other presumptive LDP (Statice bonduelli and Linum), but remained unaffected in numerous other cases
(58, 59). In Hordeum (barley)
the number of flowers (spikelets) initiated, also under long-days, was increased
both by x-ray irradiation and by the application of low auxin dosages, although
it was decreased by higher dosages (127). However, in-this case there is no
proof that we are concerned with a specific effect in floral initiation.6

The absorption of the
photoperiodic light energy.—Since variouslight actions are involved in the photoperiodic
responses of LDP and SDP, there arises the problem of the nature of the system
mediating the absorption of the light energy and the immediate changes caused
by this energy. We have at present some information about the action spectra of
the low-intensity light process of LDP (L-IV), the inhibitory light process of
SDP (S-Ill), and one piece of evidence about the nature of the last-named
process. Comparable studies on the high-intensity light processes in both
photoperiodic types are lacking. Such studies should yield material information
about the nature and the specificity of these processes, but the high energies
required make them a technically difficult job.

[281] Our information
about the action spectra of the two low-intensity light processes comes from
the work of Borthwick, Hendricks, and Parker. This work has been reviewed in
Volume I of the Annual Review of Plant Physiology (179; see also 20, 21). It has revealed that
we are dealing with a specific light-absorbing system hitherto not noted in
higher plants; that the system is identical in LDP and SDP (15, 180, 181); and
that the same system mediates the light energy in several other morphogenetic
effects of light, namely in the control of stem elongation (15) and of leaf
growth (182). In the earlier work of these authors, the extent to which the
spectra might have been modified by screening effects of other pigments was in
some doubt, but recently the same spectrum (for the control of stem elongation)
has been demonstrated in an albino mutant of Hordeum free of any of the major plant pigments (16).
Such complete absence of screening would be hard to understand if the system
which absorbs the photoperiodic light energy were localized in the
chloroplasts. This result, therefore, suggests that it is localized in the
cytoplasm.

The action spectra
obtained by Borthwick and co-workers show absorption throughout the entire
visible spectrum, but with a pronounced maximum in the red and a second, much
smaller maximum, in the blue. This finding agrees in an over-all fashion with
the results of most of the earlier work done in the field (104, 107, 223, 230,
231, 232). None of the early work had, however, been sufficiently quantitative
to reveal the difference between the photoperiodic action spectrum and that of
photosynthesis and the implication had, therefore, been that the two were
identical. Funke alone has maintained that in addition to those plants in which
the photoperiodic response is mainly determined by red light, there are three
other groups: one responding equally well to red and blue, one responding only
to red plus blue, and one responding to blue alone (66, 67, 68). Since Funke's
technique was not very satisfactory, these claims were open to some doubt. It
comes, therefore. somewhat as a surprise that recently Wassink et al. (225),
using fully reliable equipment (cf. 224), found in an LDP, a variety of Brassica
napus oleifera, a
photoperiodic action spectrum entirely different from that of Borthwick and
co-workers and similar to that of Funke's fourth type, namely, principal
effectiveness in blue and violet, and, in addition, in the near infrared (below
0.95m). It is not entirely clear, however, whether the Brassica spectrum relates to floral initiation, for the
leaf numbers which are given in one instance show comparatively small
differences and do not corroborate the clear-cut superiority of the blue-violet
range indicated by the dates of bud appearance. But the spectrum as such
remains very interesting, particularly the effectiveness of infrared.
Effectiveness of infrared (1.0-2.5m)is also reported in the SDP Perilla (166).

Borthwick and
co-workers conclude from the study of their action spectra that the absorbing
system is probably an open-chain tetrapyrrol pigment, such as a phycocyanin.
Direct attempts at identification have not yet succeeded (183). In the absence
of further indications as to the nature of the [282] pigment, its
identification promises to be a very tedious task, for a rough estimate, based
on the amount of characteristic absorption in the region of maximum
effectiveness in the albino Hordeum seedlings, indicates
that the pigment is present in exceedingly small quantities (16).

The small quantity of
the pigment may account, however, for the low-intensity character of this
photoperiodic light action. The system which absorbs the light energy for this
action will be saturated at low intensities and any light exceeding these
intensities will be wasted for the photoperiodic response.

The information about
the nature of the inhibitory light process of SDP stems from work of Harder etal. (92) on Kalanchoë. Harder
and co-workers found that the effect of light interruptions in the dark periods
of photoinduction is entirely independent of temperature. This result shows
that either the entire process S-III is a single photochemical reaction or that
any biochemical reactions associated with the light reaction require a
comparatively long time to become effective and therefore are not affected by
the temperature conditions prevailing in the short periods of the application
of light.

The movement of the
floral stimulus.—The floral
stimulus is normally transmitted through leaf, petiole, and stem tissue, but it
can also pass through root tissue
(35, 211). The translocation is entirely nonpolar (for example, 28, 31, 81,
137, 201); the stimulus may, for example, move down one branch of a plant and
up another. The movement out of the leaves can apparently take place through
the mesophyll, for it is not affected by severing the midrib at the base of the
leaf blade (33). In petioles and stems, the stimulus probably moves in the
phloem. This probability is indicated in the first place by two observations.
First, girdling, steam treatment, localized low temperature, or narcotics
treatment of stems and petioles all reduce or interrupt transmission (19, 36,
69, 228). The stimulus evidently moves in living cells. Second, in plants with
opposite leaves the stimulus stays preferentially in the stem sector adjacent
to the leaf, although this may be apparent only with low amounts of the
stimulus (nonoptimal photoinduction) or only in the early stages of
inflorescence development (83, 85, 95, 142). Since dyes introduced into the
conducting system exhibit the same behavior the movement evidently proceeds in
the vascular bundles.

The principal evidence
for the phloem transport of the stimulus, however, is furnished by another
fact. Both in localized photoinduction and in grafts, the flowering response of
the noninduced part or partner is greatly reduced by the presence of leaves
(37, 38, 39, 81, 89, 93, 98, 121, 128, 131, 133, 136, 137, 163, 165, 170, 201,
211). This inhibitory effect is present both in LDP and in SDP, although in the
former it is generally less pronounced. It may be ascribed to the generation by
the noninductive leaves of a solute stream opposed to the solute stream coming
from the induced leaves. This means, of course, that the floral stimulus is
carried along in the stream of photosynthates moving in the sieve tubes. This
explanation is very strongly suggested by the following facts: (a) the inhibitory effect is exerted only by [283] mature leaves, that
is, leaves capable of efficient photosynthesis (for example, 165); (b) it is
exerted exclusively by leaves located between the source of the stimulus and
the receiving bud (or in such cases where the induced and noninduced leaves are
located at equal distances from the bud and no other buds are present in the
plant) (38, 39, 83, 201, etc.); and (c) the inhibition is enhanced if the
youngest leaves on the receptor shoot are removed, that is, if the
effectiveness of the region attracting the solute stream is reduced (81, 128,
131, 165). The inhibitory effect of long-day leaves in SDP can be simulated by
supplying defoliated, long-day parts of the plant with sugar solutions, thus
generating an artificial solute stream opposed to the solute stream moving from
the induced leaves (40). In the SDP Perilla, the inhibitory effect of long-day leaves is
reduced if the light intensity given to these leaves is decreased (40); in the
LDP Urtica pilulifera, the inhibitory effect of short-day leaves disappears if the light
periods given to the short-day parts are reduced to 1 to 3 hr. per day (137).
In either case, the leaves apparently are no longer capable of generating an
effective solute stream. In Urtica and also in the SDP Kalanchoë (201), but apparently not in Perilla (37, 165) nor in the LDP Beta (211), inhibition is also exerted by leaves
kept in continuous darkness. Such leaves appear to act as sinks which intercept
the solute stream and sidetrack the floral stimulus.

The inhibitory effect
of leaves on the translocation of the floral stimulus can be demonstrated only
with leaves kept under the noninductive daylength conditions. It is, however,
very probably a normally occurring phenomenon. Photoinduction of a single leaf,
and even part of a leaf, in the absence of inhibiting leaves may give a
flowering response equal to that obtainable with photoinduction of the entire
intact plant (18). If the stimulus were formed in each leaf in great excess,
there should be an inverse proportionality between the minimal number of
inductive cycles and the number of leaves left on a plant. In reality, it takes
the same number of cycles to induce an intact plant and a plant defoliated to
one leaf. It thus seems as if the relative effectiveness of photoinduction
decreases with increasing size of the treated area. One reason for this
relationship is probably that the efficiency of different leaves in the
production of the floral stimulus is not the same. The sensitivity of a leaf to
photoinduction increases until full expansion and then becomes gradually
smaller (18, 160, 170); both older and immature leaves apparently produce less
stimulus than mature, but younger leaves. However, another, and quite probably
the principal, reason is that leaves which are closer to the growing point
interfere with the translocation of solutes from more distant leaves so that
the floral stimulus formed in the latter is not efficiently transmitted to the
growing points.

The only attempt to
estimate the translocation rate of the floral stimulus has yielded values
approximating 2 cm in 24 hr. in stems and 0.5 cm in 24 hr. in roots
(34, 35). These values are considerably below the translocation rates estimated
for solute movement in sieve tubes. It is, however, uncertain whether the
observed rates were optimal, since in the experiments the length [284] of the transport route
(down and up a whole stem, split lengthwise) may have been out of proportion to
the size of the supplying area (a single leaf).

The general
mechanism of photoperiodism.—The study of LDP and SDI, as
reviewed in the foregoing sections, provides us with a fairly definite idea of
the general mechanism of daylength action in floral initiation. The action
consists in adjusting the balance of several individual processes participating
in floral initiation, some of them having a promotive and some an inhibitory
character. This is an important result in itself, for such a situation is by no
means self-evident. One could have thought that in LDP, floral initiation
depended on a single light-requiring process, in SDP, on a single
light-inhibited process, and that periods of light and darkness respectively
merely delayed the attainment of the necessary effect, but did not interfere
with the effect already accumulated. In such a case, only one of the two
component conditions of daylength, either light or darkness, would be the
controlling factor, with the other acting as a mere passive interruption. The
term photoperiodism, in fact, would not be justified. But in the daylength
effect on floral initiation, light and darkness both play an active part and
their actions must be properly timed for flower formation to occur. This
daylength effect thus contains a definite element of periodicity. In LDP, however,
the action of dark periods with regard to floral initiation is a purely
inhibitory one. An actual alternation of light and dark periods is therefore
necessary for (or promotive of) floral initiation in SDP alone. One can say
that the real photoperiodic response in SDP is the induction or promotion of
floral initiation by the inductive daylength conditions, whereas in LDP the
response is the suppression or retardation of floral initiation by the
noninductive conditions.

On the basis of these
considerations, one can arrive at new and all-inclusive definitions of LDP and
SDP. LDP are plants in which the daily dark periods inhibit or delay flower
formation and SDP are plants in which the daily dark periods induce or
accelerate flower formation. These definitions also cover those plants which
lack a pronounced critical daylength (see p. 267) and therefore eluded
classifications based on the critical daylength concept. It must only be borne
in mind that in some plants daily dark periods of any duration will affect
flower formation, whereas in the majority the dark periods do not become
effective unless in excess of a certain minimum value.

These definitions have
the additional advantage of reflecting the situation in nature. The shortest
daylengths encountered anywhere in the world in the course of the growing
season are still quite in excess of the minimal light periods required for
floral initiation in SDP. The only exceptions are the so-called intermediate
plants (see p. 276) which require both a comparatively long light period and a
comparatively long dark period. These plants, therefore, flower only in a very
narrow range of natural photoperiods. But LDP, too, are capable of flower
formation with quite short light periods, provided these are not accompanied by
long dark periods (see [285] p. 272). Thus, both in
SDP and in LDP under natural conditions the factor which determines whether
and when flower formation occurs is the length of the daily dark period.

The
daylength-controlled stages of floral initiation are passed in the leaves. This
does not mean that other plant parts are entirely incapable of responding to
photoinduction or of bringing about floral initiation. We have seen that
completely defoliated Hyoscyamus plants form flowers (p. 272). Defoliated plants of Chenopodium
amaranticolor (139)and Xanthium (126) form flowers under short-day conditions.
Such cases also show that the relative strength of the different processes can
vary in different parts of a plant. In Hyoscyamus the inhibitory dark process (L-III) is limited
to leaf tissue. A comparable, although more quantitative, situation seems to
exist in Chenopodium with
regard to the inhibitory light process (S-III). If defoliated plants of Chenopodium are fed with sugar, they become capable of forming flowers under
long-day conditions [Lona (138)].If enough substrate is available, apparently the process S-Il becomes
effective in stem tissue even in the presence of long light periods. However,
if a single leaf is left on the Hyoscyamus, Chenopodium, or Xanthium plants, floral initiation
(under inductive conditions) occurs faster than in completely defoliated
individuals. There is no doubt that in intact plants photoperiodic response and
floral initiation are determined by the activity of the leaves.

We now must ask if our
knowledge of the general mechanism of daylength action enables us to formulate
a more detailed interpretation that will cover LDP and SDP at the same time. In
either response type, we have recognized a series of processes controlling the
formation of a floral stimulus. The stimulus is the same in both types.
Therefore, its production probably follows one and the same pathway in LDP and
in SDP, and a process which participates in the formation of the stimulus in a
direct and promotive manner in LDP should also be present in the SDP, and
conversely. Furthermore, the systems which mediate the light energy in the
low-intensity light process of LDP (L-IV) and in the inhibitory light process
of SDP (S-III) are also identical and it is therefore very probable that these
two processes are identical in turn and that their seemingly opposite effect
with regard to floral initiation is based on a quantitative rather than a
qualitative type of difference.

We might make, for
example, the following assumptions: (a) the formation of the floral stimulus depends on the presence of the
proper auxin level in the leaves; (b) the auxin level decreases in the course of dark periods; and (c) in
SDP the appropriate auxin level is reached only after an extended dark period,
whereas in an LDP the auxin level is lowered by such a dark period to
ineffectiveness. If this interpretation is correct, the processes L-III and
S-II and the processes L-IV and S-III would be identical. The two former would
consist in a reduction of the auxin level. Processes L-I and S-I would also be
identical, and process L-II would consist in the actual formation of the floral
stimulus in SDP as well as in LDP. [286]

It must be emphasized,
however, that the above interpretation is a mere conjecture, given as a more
tangible illustration of the kind of relationships we may have to look for
rather than as an hypothesis which is established on specific experimental
evidence. Our insight into the role of auxin in photoperiodic induction and
floral initiation is far too insufficient to draw definite conclusions, and we
do not know of an auxin-increasing light effect as assumed above. Quite
generally we must admit that our knowledge of the partial processes involved in
the formation of the floral stimulus in LDP and SDP is not yet detailed enough
and that much more experimental work will be needed before we will be in a
position to make precise comparisons between the individual processes of the
two response types.

We also cannot be
quite certain to have recognized all major processes participating in the
photoperiodic responses. While those processes of which we do know will account
in an over-all fashion for all aspects of the behavior of LDP and SDP, some
features in the kinetics of their responses are still puzzling. The dark
process of SDP (S-Il) can be nullified if the dark periods are interrupted by
very small amounts of light; yet, its effect evidently survives the very much
longer light period of the next inductive cycle. One may assume that the effect
of the dark process builds up, not in a linear fashion, but slowly at first and
then with greatly increasing rapidity; but it remains a matter for amazement
that an effect which is abolished b one minute of light after 9 hr. of darkness
can, after 15 hr. of darkness, be nullified only by light periods of somewhere
between 14 and 19 hr. duration (data after 84 and 198).

This extraordinary
change in light sensitivity can be accounted for in two basically different
ways. One can once more postulate that several consecutive changes take place
in the course of inductive dark periods and that their products differ in the.
degree of light-sensitivity. This assumption has been made by Hamner (79, 80)
who thinks, that after the beginning of the dark period changes take place
which after a definite duration of the dark period reach a threshold value, and
that then other changes set in, the result of which is much less
light-sensitive than the result of the former.

One can, however, also
assume that the efficiency of the light itself changes in the course of a dark
period. This idea has been introduced by Bünning (22 to 27), and Bünning has
made it the basis of a general hypothesis of photoperiodism, suggesting that
the changes in light efficiency are related to the so-called endogenous daily
fluctuations of activity which are known to occur in plants. The evidence which
Winning adduces in favor of this general explanation cannot be considered
convincing, two principal objections being that the course of the endogenous
periodicity has hardly been studied in plants with a clear-cut daylength
dependence of flower formation [287] and that the
endogenous periodicity occurs also in day-neutral plants and therefore cannot
be the direct basis of photoperiodic responses but can, at best, contribute an
additional element to their mechanism. The fundamental idea, however, remains,
namely, that it may not be at all the course of some light-sensitive process
involved in floral initiation that changes in a quantitative or qualitative
manner in the course of the dark period, but that it is the relative
effectiveness of light which changes in the course of this process. This idea
is supported by one piece of information, obtained in an LDP. In Hyoscyamus plants grown on 48-hr. cycles the effect of
light interruptions of the dark periods shows two maxima in the course of the
period, separated by a period of little effectiveness (50).

Nature and action
of the floral stimulus.—Our
discussion of the photoperiodic responses on LDP and SDP has been based on the
concept that photoinduction results in the formation of a transmissible floral
stimulus. This idea was generally accepted after the correlative nature of the
daylength action was clearly demonstrated. It was assumed that the stimulus is
a specific substance, a flower hormone or "florigen" [Cailahian (28,
30, 31, 32), Moskov (161)]. Recently, however, several authors have argued
against this idea (57, 138, 139, 141, 193). It has not yet been possible, in
spite of some extensive and careful attempts, to extract an active material
from photoinduced plants and to introduce it into noninduced test individuals
(81, 153). Positive effects which have been reported in a few cases either
proved irreproducible (10) or so slight that a conclusive confirmation is
urgently required (195). In view of this failure it is argued that the specific
processes of flower formation take place in the growing points and that the
effect of daylength consists not in the formation of flower-promoting
substances under the inductive conditions, but in the formation of
flower-inhibiting substances under the noninductive conditions. Some of the
authors identify these inhibitory substances with auxin (57, 193, 201) and
suggest that photoinduction involves the production of auxin antagonists (201).

Several considerations
bear on this controversy.

Nonextractability
is no conclusive proof against the existence of a flower hormone. It appears
that cellular continuity is indispensable for the transmission of the floral
stimulus because transmission in grafts occurs only after tissue union between
donor and receptor has taken place (163, 228). [Some claims to the contrary
were either not confirmed in later studies (81, 161) or are dubious for lack of
exacting controls (71).] This fact may be based on the mode of translocation of
the stimulus (along with the solute stream in sieve tubes [see p. 282]) and may
render both the isolation of the hormone and its reintroduction into test
plants virtually impossible.

The evidence
presented on p. 269 shows beyond doubt that some changes which promote floral
initiation actively and which can be communicated to noninduced plant parts or
to other plants do arise during photoinduction. Consequently, only two things
can be assumed: (1) that these changes are not of a specific nature, but
consist in the production of greater [288] amounts or of
different ratios of the gross assimilates; and (2) that in addition to the
flower-promoting changes arising under inductive conditions, there exists some
flower-inhibiting material which is formed under noninductive conditions and
prevents the promoting material from functioning.

The possibility of
fractional induction (see p. 268) is a very strong argument in favor of the
specificity of the flower-promoting changes, for if the changes were such as
mentioned in (a) 2, they
could hardly persist and accumulate through extended periods of noninductive
conditions. Fractional induction is possible only in LDP, but since the floral
stimuli of LDP and SDP are identical, this argument also holds for the latter.

In contrast to the
evidence which can be adduced in favor of the existence of transmissible
flower-promoting effects, the evidence in favor of transmissible
flower-inhibiting substances is very poor. There is no doubt that the daylength
control of flower formation involves inhibitory effects, but these effects seem
to be directed, not against their functioning, but against the formation of
flower-promoting substances. The main argument brought forward for the
existence of flower-inhibiting substances antagonistic to the floral stimulus
is the inhibitory action of noninduced leaves (see p. 282). This very action, however, can be accounted for
in terms of translocation and is thus the least specific one that could be
imagined. If it were based on the production of an inhibitory material, it
would be impossible to see why it is limited to leaves located between the
source of the floral stimulus and the responding bud. Actually, in Kalanchoë, even leaves located between an
induced leaf and the bud are not inhibitive, or are but little so, unless
inserted on the same orthostichy (93). There are at present only two cases in
which the formation of some transmissible flower-inhibiting material appears
possible, namely that of Chenopodium described on p. 285 and that of Hyoscyamus described on p. 272. In Chenopodium, intact (nondefoliated) plants do not form
flowers under long-day conditions even if fed with sugar; thus the sugar effect
evident in leafless individuals appears to be suppressed when leaves are
present (138).In Hyoscyamus, under short-day conditions,
the leaves seem to suppress the formation of the floral stimulus in the axis
tissues. However, neither case is entirely conclusive, for we may again be
dealing with translocation phenomena. In Chenopodium, the solute stream proceeding from mature leaves
may prevent the comparatively small amounts of floral stimulus formed in the
stem of sugar-supplied plants from reaching the growing point; in Hyoscyamus, the effect may be the result
of the diversion of some material from' the axis into the leaves (see p. 274).

The idea that
auxin is a flower-inhibiting agent is based on the inhibition of flower
formation by applied auxin and its promotion by a lowered auxin level in the
plant (see p. 279). But we
have seen evidence that the auxin level enters into only one specific phase of
photoinduction, at least in the case of SDP, and that this phase is once again
concerned with the formation, and not with the functioning, of the floral
stimulus. If the inhibitory effect of noninduccd leaves were based on auxin
production (201),one would [289] have to assume that
the auxin is transported to the induced leaves. This transportation would in
numerous cases necessitate an upward movement. Auxin, however, is known to move
only downward.

If photoinduction
consisted in the production of an auxin antagonist which is transported to the
growing points, floral initiation should be induced under noninductive
conditions by antiauxin treatment of the growing points. This obvious
experiment has apparently not been made, but experiments in which the whole
plants were treated do not support the idea. Since, however, there is no
critical evidence whatsoever that the auxin concentration in the growing points
has something to do with floral initiation, the entire idea is questionable.
Besides, it is difficult to comprehend why an antiauxin could not be extracted
from induced plants.

Two more general
considerations also support the idea that the floral stimulus is a specific
agent promoting floral initiation in a direct manner. One is the absence of any
aftereffect of noninductive conditions. If these conditions acted by causing
the formation of flower-inhibiting substances, one would expect that the longer
a plant is maintained under the noninductive daylength, the more resistant it
will be to photoinduction. In reality, the responsiveness to photoinduction
increases either continuously (in such plants which ultimately flower even
under extreme noninductive conditions) or until a final optimal level is
reached.

The second
consideration is based on the mode of action of daylength and it is also
important for the general understanding of the role of daylength in the control
of floral initiation. Floral initiation is an all-or-none event. Therefore, any
factor or process which controls this event in a rather direct manner should
likewise, have an all-or-none mode of action. If, on the contrary, a factor has
a clearly quantitative effect, one can assume that some further processes
intervene between its primary effect and floral initiation. Thus, if daylength
were controlling floral initiation through the production of assimilates or of
an auxin antagonist, whereas the specific processes of floral initiation took
place in the growing points, one would expect photoinduction to have a
quantitative effect. The effect of photoinduction in floral initiation is,
however, pronouncedly qualitative, that is, of the all-or-none type. In Hyoscyamus, floral initiation takes place after the plant
has received a definite number of photoinductive cycles, and cycles in excess
of this number have no additional accelerating effect (123). Similarly, in
short-day strains of Gossypium hirsutum, the first flowers are formed at approximately the same node,
regardless of whether photoinduction was optimal or at the bare minimum of
effectiveness (121). It appears that the floral stimulus is accumulated in the
plant until a threshold concentration necessary for the differentiation of a
floral primordium is attained; then the primordium is formed whether or not the
production of the stimulus continues.

In summary, all
available evidence is in agreement with the concept that the result of
photoinduction is the appearance of a specific flower hormone which controls
floral initiation in a direct and positive (promotive) manner. [290] noninductive daylength
conditions interfere with the formation and not with the functioning of the
hormone.